1. Field of the Invention
The present invention relates in general to the field of reproduction technology, and particularly to the field of printing technology wherein the scanning is carried out on the basis of already rastered masters for later reproduction.
2. Prior Art
In electron-optical scanning of rastered masters on scanners and graving devices, the problem exists that a disruptive Moire arises due to the superimposition of the master raster on the one hand and the raster added on the other hand during recording in the machine. During the scanning, a small image point from a uniformly illuminated master location is imaged by means of a diaphragm onto the sensitive surface of an electronic light receiver which generates the electronic signal for the further processing and, finally, the recording.
When the size of the imaged spot is not large in comparison to the raster spacing of a rastered master, then the repetition frequency of the raster points running or moving by the scanning optics during the scanning usually appears in the output signal of the light receiver. This unwanted frequency interferes with the frequency which is added to the brightness signal for rastering the recording.
A method for avoiding Moire, which occurs during the above-mentioned scanning, is disclosed in U.S. patent application Ser. No. 246,160, filed Mar. 23, 1981, whose disclosure is incorporated by reference thereto. The method of this above-mentioned application prevents or suppresses the raster structure of the master and thus the undesired freqency in the image signal by placing the scanning optics out of focus by a certain amount. In order to avoid the loss of focus which occurs, the loss of sharpness is at least partially compensated in this method with the assistance of an electronic unsharpmasking.
A corrective method is known from the handbook, Agfa-Gevaert, Grafisches Material, Halbtonfotografie, 12, 1969, pages 132-137. In this method, the master is de-rastered by means of diffraction at diaphragms. In this method, the diaphragms are designed and employed for the simultaneous imaging of the entire master. Thus, these diaphragms are not suitable for the point-wise and line-wise scanning of a master as utilized in electronic reproduction devices.